The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and, in particular, to a form of cutting structure for use on such bits.
Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank, inner passages for supplying drilling fluid to the face of the bit, and a plurality of cutting structures mounted on the face of the bit.
Each cutting structure includes a front layer formed of superhard material and having a front cutting face defining a cutting edge, and a backing structure formed of less hard material. For example, the superhard material may be polycrystalline diamond and the backing structure may be formed of cemented tungsten carbide.
The backing structure may include a backing layer or less hard material to which the front layer is bonded. The backing layer may then constitute the while of the backing structure, or the backing structure may further include a carrier on which the cutting element (comprising the front layer and backing layer) is mounted, for example by bonding.
Cutting elements are also available in the form of a unitary layer of polycrystalline diamond, such unitary cutting elements having the advantage that they may be thermally stable at the temperatures used to form some types of bit body. In this case there is no backing layer and the carrier on which the cutting element is mounted will constitute the whole of the backing structure.
Where a carrier is provided, the cutting element is usually bonded, for example by brazing, to the carrier which may be in the form of a stud of tungsten carbide which is received and located in a socket in the bit body. The bit body may be machined from steel or may comprise an infiltrated matrix material formed by a powder metallurgy process. The construction of the bit body and the method of mounting the cutting structures thereon do not form part of the present invention and will not therefore be described in detail.
The two or three layer arrangement of the cutting structure provides a degree of self-sharpening since, in use, the less hard material of the carrier and/or backing layer wears away more easily than the harder cutting face of the cutting element.
However, although this self-sharpening effect is advantageous, a wear flat forms on the cutting structure in the course of its use, and the necessary configuration of the cutting structure is usually such that the wear flat increases in area with continuing use, thus resulting in increasing frictional resistance to the rotation of the drill bit and increasing heat effect, which may lead to failure of a cutting element or its bond to a carrier or bit body.
The present invention sets out to provide an arrangement whereby, as the cutting structure wears in use, the configuration of the structure changes so that the wear flat does not continually increase in area but periodically becomes reduced in area.